1. Field of the Invention
This invention relates to an optical communication system and an optical receiver used in the optical communication system.
2. Related Prior Art
An optical communication system comprises an optical transmitter, an optical path and an optical receiver. The optical transmitter transmits signal light into the optical path, the optical path propagates the signal light and the optical receiver receives the propagated light. The optical communication system has an ability to send a huge amount of information with a high speed. To secure a quality of the propagated signal light, it is required to reduce an absolute dispersion accumulated from the transmitter to the receiver on the optical path.
On the other hand, typical optical fiber used in the optical communication system shows a zero-dispersion wavelength around 1.3 μm band, while a finite dispersion of +16 ps/nm/km around 1.55 μm band where the optical communication system is performed. In the case that the optical path is constructed by such conventional optical fiber alone, the large dispersion will cause to deteriorate of the quality of the transmission signal.
To overcome the deterioration of the transmission quality due to large accumulated dispersion, a means for correcting the dispersion is placed on the optical path. The dispersion-correcting means has a negative dispersion to compensate the finite dispersion, which is a positive amount, of the optical path. The dispersion correcting means is a dispersion-correcting fiber, for example. To adjust a ratio of a length of the single mode optical fiber, which is used in the optical path, to that of the dispersion-correcting fiber, the accumulated dispersion can be reduced, whereby enhancing the quality of the transmitted signal light.
However, the single mode fibers that are used in the optical path have various lengths and the dispersion attributed thereto show various values. Also, dispersion of the dispersion correcting-fibers has various values. Further, the optical transmitter and the optical receiver have respective tolerance for the dispersion. Therefore, the optical communication system should be designed by taking the above mentioned factors into account, which raises a designing cost and a management cost of the communication system.
Further, Since the dispersion-correcting fiber has a large transmission loss, several optical amplifiers to amplify the signal light must be installed on the optical path, which raises the cost for construction of the optical system. Moreover, because optical amplifiers accompany optical noises, the cost is required for reduce such optical noises to maintain the quality of the signal light. Also, distortion due to a non-linear effect in the optical fiber causes the deterioration of the transmitted signal light, because an output of the optical amplifier becomes so high that the non-linear effect of the optical fiber must be taken into account, which raises the cost of the communication system.